I would say that depends on your target group and the product you want to sell. If your target group is active on Facebook I would start with Facebook. Ultimately only testing your case will give you the "correct" answer...Do a test with around 50 conversions on both systems and see which was cheaper. Then go with that. Of course it could mean that you have done either system wrongly, but if you are not going to invest much into PPC education, this is the easiest way to get a feeling for both...